Unable to feel happy. Is it depression?

I don't remember the last time I felt happy. I don't know why I just don't feel happy. It's not new, this has been going on since a long time...it's been like this since the time I was 13 or 14. I just feel that things aren't going right in my life. I also feel that things have never worked in my favour, or the way I wanted them to, but that's okay. I've just graduated this year and I've taken a year off to prepare for an exam which requires me to put my heart and soul into the preparation. I really want to study hard, but I just don't feel like studying. I just feel like sleeping all day. I've also put on weight. I don't really like most of the people, so, I don't have friends but I am sure that this is not the reason why I am unhappy. But let me also make it clear that I've never felt like killing myself. In fact, I want to live my life. Is it depression? I think I have mild depression.